The present invention relates generally to optical elements, and more specifically to optical elements having high damage resistance and tolerance.
An example optical element requiring high damage resistance and tolerance is an optical limiter. Optical limiters are generally transparent optical elements that block, or limit to a maximum intensity, the transmission of incident light at specific wavelengths. A primary use for optical limiters will be for protection against laser radiation.
Optical limiters may be based on a number of different physical processes, such as scattering, absorption or reflection.
Optical limiters based on absorptive processes can be optimized by controlling the geometrical location of an absorbing species dopant both longitudinally and transversely inside the limiter so that, for example, greater amounts of the absorbing species dopant are located at an expected focal region for incoming laser irradiation. Such optimized optical limiters, however, are subject to optically induced damage inside that focal region. In solid matrices, the damage mechanism may be thermomechanical or thermochemical. Thermomechanical damage occurs by conversion of laser energy to heat at the site of an absorbing inclusion. The rapid heating process at the inclusion causes a thermomechanical fracture to occur in the surrounding matrix or host. The resulting small fracture site is a source for scattering and results in an irreversible damage site.
Prior art optical limiters commonly use solid polymeric plastic host matrices with appropriate, usually light absorbing, dopants to produce optimized optical limiters. An example solid polymeric plastic host material is polymethylmethacrylate (PMMA). Unfortunately, the susceptibility of these plastic-based optical elements to damage reduces the range over which this type of solid limiter may be used.
One method for overcoming the problem of permanent damage in a host matrix is to use a liquid limiter based on a chromophore dopant dissolved in a solvent. A chromophore is that portion of a dye molecule that gives it its color and is usually the most fragile part of the molecule. The advantage of liquid limiters is that they are damage tolerant and can undergo “self-healing” once damaged. The self-healing process occurs when the damage site, usually in the form of a bubble, floats away and new solution takes its place. A primary disadvantage of liquid limiters is that their performance cannot be optimized by control of the geometrical placement of absorbing species since liquids cannot preserve a concentration distribution. Liquids are also subject to leaking and other types of failures.
Returning to rigid host matrix materials, there are other rigid host matrix materials that exhibit increased damage resistance compared to other materials.
Unfortunately, despite the high damage resistance of some rigid host matrix materials, their operational range as provided through damage resistance is still limited. Further, they exhibit little damage tolerance. Damage tolerance is not the same as damage resistance. Damage resistance indicates the ability to accept higher radiation levels without any apparent effect. Damage resistance can only be increased to some level and then irreversible permanent damage will occur. Damage tolerance, however, as exhibited by, for example, liquid optical limiters, indicates the desirable ability to allow damage to occur temporarily and then subsequently heal. While there is a limit, of course, to damage tolerance before irreversible damage will occur, adding a damage tolerance capability to damage resistance should substantially increase the operational range of optical limiters.
Thus it is seen that there is a need for optimized optical limiters having increased damage resistance and a further need for optimized optical limiters having increased damage tolerance.
Another example optical element requiring high damage resistance and tolerance are the gain or lasing media in solid state dye lasers, particularly dye-doped polymer host materials. Such solid state dye lasers are an attractive alternative to more common liquid dye lasers (which use complex organic dyes such as rhodamine 6G in liquid solution or suspension as lasing media), providing the tuning and other advantages of liquid dye lasers without such problems as sealing and the size and complexities of pumping. Polymer host matrices are particularly attractive because, among other reasons, they can be easily doped with dyes at high concentrations. Unfortunately, solid state dye lasers using polymer host materials are severely limited in power output by low damage thresholds.
U.S. Pat. No. 5,610,932 to Kessler et al., which is incorporated by reference into this description, discloses a solid state dye laser host made of a gel material. The use of a gel material appears to result in “‘self-healing’ after photobleaching due to dye migration,” thus avoiding many of the disadvantages of liquid dye lasers without forfeiting as much power output as in most solid state dye lasers. The Kessler et al. invention nevertheless will not be able to achieve the same power outputs as liquid dye lasers.
Thus it is seen that there is also a need for solid state dye lasers having increased damage resistance and tolerance and thus capable of higher power outputs.
Many other, if not most, optical elements will benefit from new apparatus and methods for increasing damage resistance and tolerance to the transmission of light energy through those optical elements.
It is, therefore, a principal object of the present invention to provide optical elements having high damage resistance and tolerance.
It is a feature of the present invention that it provides a nonuniform distribution of dopants, such as dopants for an optical limiter or lasing media for a solid state dye laser, inside a host material.
It is another feature of the present invention that it adds gradient mechanical properties to optical elements.
It is an advantage of the present invention that it has ability to form and preserve a dopant concentration distribution not possible in liquids.
It is another advantage of the present invention that it resists agglomeration of particulate dopants, thus providing longer shelf life and more durability against environmental extremes than liquid, solvent-based optical elements.
It is a further advantage of the present invention that it allows both the placement of appropriate dopants and the gradient mechanical properties of a host matrix material to be optimized for maximum performance.
It is a still further advantage of the present invention that it is safer than liquid optical elements where a glass cell may fracture with a resultant spill of hazardous solvents.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.